Corrosion inhibition method for inorganic acidic media

ABSTRACT

Methods for preventing, inhibiting, or reducing metal (e.g. mild steel) corrosion in aqueous media utilizing a water-soluble formulation having a polyurea pre-dissolved in a polar aprotic solvent are described. The polyurea contains reacted units of a diisocyanate and a diaminoalkane. The effectiveness of the methods is demonstrated by corrosion inhibition efficiency and corrosion rate of metallic substrates in aqueous acidic environments using the water-soluble formulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.16/361,355 now allowed, having a filing date of Mar. 22, 2019.

STATEMENT OF FUNDING ACKNOWLEDGEMENT

This project was supported by the Deanship of Scientific Research (DSR)at King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabiaunder project No. NUS15107/8.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to methods of preventing or reducingmetal corrosion in aqueous media using a water-soluble compositioncontaining polyurea pre-dissolved in a polar aprotic solvent.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

The use of acidic solutions for scaling control and removal is commonpractice in many industrial processes. However, these acidic solutionsattack metallic structures leading to metallic corrosion [N. Ahmad, A.G. MacDiarmid, Inhibition of corrosion of steels with the exploitationof conducting polymers, Synthetic Metals, 78 (1996) 103-110; and S. K.Shukla, M. A. Quraishi, R. Prakash, A self-doped conducting polymer“polyanthranilic acid”: An efficient corrosion inhibitor for mild steelin acidic solution, Corrosion Science, 50 (2008) 2867-2872]. Corrosionof metallic structures constitutes a significant portion of the totalcost incurred in oil and gas producing companies. It often leads toincidents including pollution of water and other natural resources,shutdown of oil production facilities, and even loss of life [L. A. C.J. Garcia, C. J. B. M. Joia, E. M. Cardoso, O. R. Mattos,Electrochemical methods in corrosion on petroleum industry: laboratoryand field results, Electrochimica Acta, 46 (2001) 3879-3886].

Organic molecules containing oxygen, sulfur or nitrogen heteroatoms, aswell as those having a delocalized π-electron system, have beeneffectively used as organic corrosion inhibitors [A. Zeino, I.Abdulazeez, M. Khaled, M. W. Jawich, I. B. Obot, Mechanistic study ofpolyaspartic acid (PASP) as eco-friendly corrosion inhibitor on mildsteel in 3% NaCl aerated solution, Journal of Molecular Liquids, 250(2018) 50-62; and L. O. Olasunkanmi, I. B. Obot, M. M. Kabanda, E. E.Ebenso, Some Quinoxalin-6-yl Derivatives as Corrosion Inhibitors forMild Steel in Hydrochloric Acid: Experimental and Theoretical Studies,The Journal of Physical Chemistry C, 119 (2015) 16004-16019]. Thesemolecules demonstrate anticorrosion properties through displacement ofwater molecules on the metal surface and formation of a protectivelayer, thus isolating the metal from corrosive medium [A. Ongun Yüce, B.Doğru Mert, G. Kardaş, B. Yazιcι, Electrochemical and quantum chemicalstudies of 2-amino-4-methyl-thiazole as corrosion inhibitor for mildsteel in HCl solution, Corrosion Science, 83 (2014) 310-316]. In thepast few years, organic polymers have attracted great attention incorrosion inhibition research because of their high chemical stability,large surface coverage, cost effectiveness, and high corrosioninhibition efficiencies at relatively low concentrations compared tosmall organic molecules [B. Ramaganthan, M. Gopiraman, L. O.Olasunkanmi, M. M. Kabanda, S. Yesudass, I. Bahadur, A. S. Adekunle, I.B. Obot, E. E. Ebenso, Synthesized photo-cross-linking chalcones asnovel corrosion inhibitors for mild steel in acidic medium:experimental, quantum chemical and Monte Carlo simulation studies, RSCAdvances, 5 (2015) 76675-76688; R. Baskar, D. Kesavan, M. Gopiraman, K.Subramanian, Corrosion inhibition of mild steel in 1.0M hydrochloricacid medium by new photo-cross-linkable polymers, Progress in OrganicCoatings, 77 (2014) 836-844; and Y. Ren, Y. Luo, K. Zhang, G. Zhu, X.Tan, Lignin terpolymer for corrosion inhibition of mild steel in 10%hydrochloric acid medium, Corrosion Science, 50 (2008) 3147-3153, eachincorporated herein by reference in their entirety]. The presence ofvarious functional groups in polymers enables them to form stronginteractions with the metal ions. These interactions occupy a largepercentage of surface area on the metal thus preventing furtherdissolution and corrosion [Y. Ren, Y. Luo, K. Zhang, G. Zhu, X. Tan,Lignin terpolymer for corrosion inhibition of mild steel in 10%hydrochloric acid medium, Corrosion Science, 50 (2008) 3147-3153,incorporated herein by reference in its entirety].

Because of its high thermal and chemical resistance, good abrasionresistance, structural enhancement, waterproofness, and structuraldiversity, polyurea is commonly used as a protective coating againstmetallic corrosion [K. W. Allen, S. M. Smith, W. C. Wake, A. O. vanRaalte, The concept of an endurance limit for adhesive joints,International Journal of Adhesion and Adhesives, 5 (1985) 23-32,incorporated herein by reference in its entirety]. A number of studieson the corrosion inhibitive performance of polyurea coatings have beenreported. Guoqiang et al. reported using a oligoaniline pendantgroup-grafted polyurea as a corrosion protection coating on cold rolledsteel [G. Qu, F. Li, E. B. Berda, M. Chi, X. Liu, C. Wang, D. Chao,Electroactive polyurea bearing oligoaniline pendants: Electrochromic andanticorrosive properties, Polymer, 58 (2015) 60-66, incorporated hereinby reference in its entirety]. The electroactive polyurea (EPU) coatingsshowed excellent corrosion protection properties with enhanced thermalstability and electrochemical properties. Maia et al. reported thesynthesis of polyurea microcapsules loaded with 2-mercaptobenzothiazole(MBT) for corrosion protection of aluminum 2024 alloy [F. Maia, K. A.Yasakau, J. Carneiro, S. Kallip, J. Tedim, T. Henriques, A. Cabral, J.Venâncio, M. L. Zheludkevich, M. G. S. Ferreira, Corrosion protection ofAA2024 by sol-gel coatings modified with MBT-loaded polyureamicrocapsules, Chemical Engineering Journal, 283 (2016) 1108-1117,incorporated herein by reference in its entirety]. An improved adhesionenhancement and corrosion protective properties was found. Feng et al.reported the preparation of a polyurea and polyimide copolymer as aprotective coating for 2024-T3 aluminum alloy [L. Feng, J. O. Iroh,Corrosion resistance and lifetime of polyimide-b-polyurea novelcopolymer coatings, Progress in Organic Coatings, 77 (2014) 590-599,incorporated herein by reference in its entirety]. The protectivecoating was found to be highly hydrophobic with a water contact angle of110°, a remarkably low surface energy, and improved corrosion protectiveproperties. A few other studies on the corrosion inhibitive propertiesof polyurea coatings have been documented [V. V. Gite, P. D. Tatiya, R.J. Marathe, P. P. Mahulikar, D. G. Hundiwale, Microencapsulation ofquinoline as a corrosion inhibitor in polyurea microcapsules forapplication in anticorrosive PU coatings, Progress in Organic Coatings,83 (2015) 11-18; and A. Kakaroglou, M. Domini, I. De Graeve,Encapsulation and incorporation of sodium molybdate in polyurethanecoatings and study of its corrosion inhibition on mild steel, Surfaceand Coatings Technology, 303 (2016) 330-341, each incorporated herein byreference in their entirety]. In all the studies conducted so far,polyurea has been utilized for coating metallic structures primarily dueto its lack of solubility in aqueous medium. However, soluble corrosioninhibitors may be more advantageous than protective coatings incorrosion inhibition applications [B. D. B. Tiu, R. C. Advincula,Polymeric corrosion inhibitors for the oil and gas industry: Designprinciples and mechanism, Reactive and Functional Polymers, 95 (2015)25-45, incorporated herein by reference in its entirety] althoughpreparation and anti-corrosion properties of water-soluble polyureacompositions have not been reported.

In view of the forgoing, one objective of the present disclosure is toprovide methods of utilizing a water-soluble formulation containingpolyurea pre-dissolved in a polar aprotic solvent for inhibitingcorrosion of metallic substrates in an acidic aqueous environment suchas that commonly found in industrial processes such as scale removal,acid pickling, and well acidification.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodof inhibiting corrosion of a metallic substrate in contact with anaqueous corrosive medium. The method involves introducing a formulationcontaining a polyurea pre-dissolved in a polar aprotic solvent into theaqueous corrosive medium in contact with the metallic substrate, wherein(i) the formulation is devoid of epoxy ester resin, (ii) the polyureacomprises reacted units of a diaminoalkane and a diisocyanate, and (iii)the polar aprotic solvent is at least one selected from the groupconsisting of N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide, and 1,3-dimethyl-2-imidazolidinone.

In one embodiment, the diaminoalkane is at least one selected from thegroup consisting of ethylenediamine, 1,2-diaminopropane,1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, and1,6-diaminohexane.

In one embodiment, the diaminoalkane is 1,4-diaminobutane.

In one embodiment, the diisocyanate is toluene 2,4-diisocyanate,toluene-2,6-diisocyanatethe, or both.

In one embodiment, the diisocyanate is a mixture of toluene2,4-diisocyanate and toluene-2,6-diisocyanate.

In one embodiment, a molar ratio of the toluene 2,4-diisocyanate to thetoluene-2,6-diisocyanate is in a range of 1:1 to 10:1.

In one embodiment, the polyurea is in the form of porous microparticles.

In one embodiment, the porous microparticles are spherical.

In one embodiment, the polar aprotic solvent is N-methyl-2-pyrrolidone.

In one embodiment, a volume ratio of the polar aprotic solvent to theaqueous corrosive medium is in a range of 1:80 to 1:1,000.

In one embodiment, the polyurea is introduced into the aqueous corrosivemedium at a concentration of 5-500 ppm.

In one embodiment, the metallic substrate contains steel.

In one embodiment, the metallic substrate contains carbon steel.

In one embodiment, the aqueous corrosive medium comprises at least oneacid selected from the group consisting of hydrochloric acid, sulfuricacid, nitric acid, hydrofluoric acid, acetic acid, and formic acid.

In one embodiment, the aqueous corrosive medium comprises hydrochloricacid.

In one embodiment, the aqueous corrosive medium has a pH of 5 or below.

In one embodiment, the formulation is soluble in water.

In one embodiment, the aqueous corrosive medium has a temperature in arange of 4-80° C.

In one embodiment, the method has a corrosion inhibition efficiency in arange of 70-99.9%.

In one embodiment, the method imparts a corrosion rate in a range of0.005-1.1 millimeter penetration per year (mmpy) to the metallicsubstrate.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a synthesis scheme for polyurea.

FIG. 2A is a ¹³C NMR spectrum of a mixture of toluene 2,4-diisocyanate(2,4-TDI), toluene-2,6-diisocyanate (2,6-TDI), and 1,4-diaminobutane indeuterated DMSO (DMSO-d₆).

FIG. 2B is a ¹³C NMR spectrum of polyurea in DMSO-d₆.

FIG. 3A is a ¹H NMR spectrum of a mixture of toluene 2,4-diisocyanate(2,4-TDI), toluene-2,6-diisocyanate (2,6-TDI), and 1,4-diaminobutane inDMSO-d₆.

FIG. 3B is a ¹H NMR spectrum of polyurea in DMSO-d₆.

FIG. 4A is a FT-IR spectrum of polyurea.

FIG. 4B is a thermal gravimetric analysis (TGA) curve of polyurea.

FIG. 5A shows a structural geometry and frontier orbitals ofN-methyl-2-pyrrolidone.

FIG. 5B shows a structural geometry and frontier orbitals of polyurea.

FIG. 6 is an overlay of open circuit vs time curves of mild steel at 25°C. (298 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 7A is an overlay of Tafel polarization curves of mild steel at 25°C. (298 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 7B is an overlay of Tafel polarization curves of mild steel at 30°C. (303 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 7C is an overlay of Tafel polarization curves of mild steel at 35°C. (308 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 7D is an overlay of Tafel polarization curves of mild steel at 40°C. (313 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 7E is an overlay of Tafel polarization curves of mild steel at 50°C. (323 K) in 1.0 M HCl aqueous solution in the absence (blank) orpresence of polyurea at 20, 50, and 100 ppm, respectively.

FIG. 8A is an overlay of Nyquist plots of mild steel at 25° C. (298 K)in 1.0 M HCl aqueous solution in the presence of polyurea at 20, 50, and100 ppm, respectively.

FIG. 8B is a Nyquist plot of mild steel at 25° C. (298 K) in 1.0 M HClaqueous solution in the absence of polyurea (blank).

FIG. 8C is an overlay of Nyquist plots of mild steel at 40° C. (313 K)in 1.0 M HCl aqueous solution in the presence of polyurea at 20, 50, and100 ppm, respectively.

FIG. 8D is a Nyquist plot of mild steel at 40° C. (313 K) in 1.0 M HClaqueous solution in the absence of polyurea (blank).

FIG. 8E is an overlay of Bode plots of mild steel at 25° C. (298 K) in1.0 M HCl aqueous solution in the absence (blank) or presence ofpolyurea at 20, 50, and 100 ppm, respectively.

FIG. 8F is an overlay of Bode plots of mild steel at 40° C. (313 K) in1.0 M HCl aqueous solution in the absence (blank) or presence ofpolyurea at 20, 50, and 100 ppm, respectively.

FIG. 9 is an overlay of FT-IR spectra of polyurea and adsorbed polyureaon mild steel after introducing polyurea according to the presentmethod.

FIG. 10A is a scanning electron microscopy (SEM) micrograph showing amild steel surface before immersing in 1.0 M HCl aqueous solution.

FIG. 10B is a SEM micrograph showing a mild steel surface afterimmersing in 1.0 M HCl aqueous solution in the absence of polyurea at25° C. for 24 hours.

FIG. 10C is a SEM micrograph showing a mild steel surface afterimmersing in 1.0 M HCl aqueous solution in the presence of polyurea at25° C. for 24 hours.

FIG. 11A shows high resolution N1s X-ray photoelectron spectroscopy(XPS) spectra of mild steel after immersing in 1.0 M HCl aqueoussolution in the presence of polyurea.

FIG. 11B shows high resolution O1s XPS spectra of mild steel afterimmersing in 1.0 M HCl aqueous solution in the presence of polyurea.

FIG. 11C shows high resolution C1s XPS spectra of mild steel afterimmersing in 1.0 M HCl aqueous solution in the presence of polyurea.

FIG. 11D shows high resolution Fe2p XPS spectra of mild steel afterimmersing in 1.0 M HCl aqueous solution in the presence of polyurea.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

As used herein, the words “a” and “an” and the like carry the meaning of“one or more”. Within the description of this disclosure, where anumerical limit or range is stated, the endpoints are included unlessstated otherwise. Also, all values and subranges within a numericallimit or range are specifically included as if explicitly written out.

As used herein, the words “substantially similar”, “substantiallyidentical”, “approximately”, or “about” may be used when describingmagnitude and/or position to indicate that the value and/or positiondescribed is within a reasonable expected range of values and/orpositions. For example, a numeric value may have a value that is ±1% ofthe stated value (or range of values), ±2% of the stated value (or rangeof values), ±5% of the stated value (or range of values), ±10% of thestated value (or range of values), or ±15% of the stated value (or rangeof values).

As used herein, the terms “compound”, “product” and “monomer” are usedinterchangeably, and are intended to refer to a chemical entity, whetherin the solid, liquid or gaseous phase, and whether in a crude mixture orpurified and isolated.

According to a first aspect, the present disclosure relates to a methodof inhibiting corrosion of a metallic substrate in contact with anaqueous corrosive medium. The method involves introducing a formulationcontaining a polyurea pre-dissolved in a polar aprotic solvent into theaqueous corrosive medium in contact with the metallic substrate.Preferably, the polar aprotic solvent is at least one selected from thegroup consisting of N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide, and 1,3-dimethyl-2-imidazolidinone.

As used herein, monomers are molecules which can undergo polymerization,thereby contributing constitutional repeating units to the structures ofa macromolecule or polymer matrix. The process by which monomers combineend to end to form a polymer matrix is referred to herein as“polymerization”. As used herein, “crosslinking”, “cross-linking”,“crosslinked”, “cross-linked”, a “crosslink”, or a “cross-link” refersto polymer matrixes containing branches that connect polymer chains viabonds that link one polymer chain to another. The crosslink may be anatom, a group of atoms, or a number of branch points connected by bonds,groups of atoms, or polymer chains. A crosslink may be formed bychemical reactions that are initiated by heat, pressure, radiation,change in pH, etc. with the presence of at least one crosslinkingmonomer having more than two extension points, which is a monomer havingmore than two reactive sites (i.e. a poly-functional monomer).

As used herein, the term “polyurea” refers to products of apolymerization reaction of a suitable di-functional or poly-functionalisocyanate monomer with a suitable di-functional or poly-functionalamine monomer. The polymerization provides urea or carbamide linkages(—NH—CO—NH—) between isocyanate and amine moieties of the polyureanetwork. Preferably, the isocyanate and the amine monomers used hereinare di-functional, each having two reactive sites (e.g. isocyanategroups, amino groups). In one or more embodiments, the polyurea of thepresent disclosure comprises reacted units of a diamine and adiisocyanate.

As used herein a “copolymer” refers to a polymer derived from more thanone species of monomer and are obtained by “copolymerization” of morethan one species of monomer. Copolymers obtained by copolymerization oftwo monomer and/or oligomer species may be termed bipolymers, thoseobtained from three monomers may be termed terpolymers and thoseobtained from four monomers may be termed quarterpolymers, etc. In someembodiments, the polyurea of the present disclosure is a terpolymer, forexample a terpolymer obtained from reaction between a diisocyanate and amixture of two diamines with different chemical structures. In apreferred embodiment, the polyurea of the present disclosure is abipolymer obtained from reaction between a diisocyanate and a diamine.

The diisocyanate of the present disclosure may be a compound having twoisocyanate groups, in particular, an isocyanate compound used for thesynthesis of polyurea polymers. In a preferred embodiment, thediisocyanate is aromatic in nature by having one or more optionallysubstituted aromatic structures. The aromatic structure can bemonocyclic or polycyclic. Examples of aromatic structures include, butare not limited to, benzene, naphthalene, anthracene, phenanthrene,pyrene, and biphenyl. Exemplary aromatic diisocyanates include, but arenot limited to, toluene 2,4-diisocyanate, toluene-2,6-diisocyanatethe,1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate,1,3-bis(1-isocyanato-1-methylethyl)benzene,1-chloromethyl-2,4-diisocyanatobenzene, 4-chloro-6-methyl-1,3-phenylenediisocyanate, 4,4′-methylenebis(phenyl isocyanate),2,4′-methylenebis(phenyl isocyanate), 2,2′-methylenebis(phenylisocyanate), m-xylylene diisocyanate, and 1,5-naphthalene diisocyanate.It is equally envisaged that the currently disclosed method may beadapted to include aliphatic diisocyanates such as isophoronediisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate,pentamethylene diisocyanate, hexamethylene diisocyanate, 2,3-butylenediisocyanate, 1,3-butylene diisocyanate, 2,4,4-trimethylhexamethylenediisocyanate, 4,4′-dicyclohexylmethane diisocyanate, dodecamethylenediisocyanate, 1,3-cyclopentane diisocyanate, 1,3-cyclohexanediisocyanate, 1,4-cyclohexane diisocyanate, methyl-2,4-cyclohexanediisocyanate, and methyl-2,6-cyclohexane diisocyanate. Alternatively,polyisocyanate such as 2,4,6-triisocyanate toluene, 1,3,5-triisocyanatebenzene, 4,4′-triphenylmethane triisocyanate may be used in addition toor in lieu of the diisocyanate.

In one or more embodiments, the diisocyanate of the present disclosureis toluene 2,4-diisocyanate, toluene-2,6-diisocyanatethe, or both. In apreferred embodiment, the diisocyanate is a mixture of toluene2,4-diisocyanate and toluene-2,6-diisocyanate. Preferably, a molar ratioof the toluene 2,4-diisocyanate to the toluene-2,6-diisocyanate of themixture is in a range of 1:1 to 10:1, 1.5:1 to 9:1, 2:1 to 8:1, 2.5:1 to7:1, 3:1 to 6:1, 3.5:1 to 5:1, or about 4:1. The mixture of toluene2,4-diisocyanate and toluene-2,6-diisocyanate may be available fromcommercial vendors including, without limitation, Sigma Aldrich, AlfaAesar, and TCI America.

In a preferred embodiment, diamines of the present disclosure arealiphatic diaminoalkanes including, but not limited to, ethylenediamine,1,2-diaminopropane, 1,3-diaminopropane, 1,4-diaminobutane,1,5-diaminopentane, 1,6-diaminohexane, 1,12-diaminododecane,2,5-diamino-2,5-dimethylhexane, trimethyl-1,6-hexane-diamine,piperazine, 1,4-diaminocyclohexane, isophoronediamine,N-cyclohexyl-1,3-propanediamine, bis-(4-amino-cyclohexyl)methane, andbis-(4-amino-3-methyl-cyclohexyl)-methane. It is equally envisaged thatthe currently disclosed method may be adapted to include polyamines suchas diethylenetriamine, triethylenetetraamine, tetraethylenepentamine,pentaethylenehexamine, dipropylenetriamine, tripropylenetetraamine, bis-(3-aminopropyl)amine in addition to or in lieu of the aforementioneddiaminoalkanes. Alternatively, suitable aromatic diamines may be used.Non-limiting examples of aromatic diamines include o-, m- andp-phenylenediamine, 1,2-diamino-3-methylbenzene,1,3-diamino-4-methylbenzene(2,4-diaminotoluene),1,3-bisaminomethyl-4,6-dimethylbenzene, 2,4- and2,6-diamino-3,5-diethyltoluene, 1,4- and 1,6-diaminonaphthalene, 1,8-and 2,7-diaminonaphthalene, bis-(4-amino-phenyl)-methane,2,2-bis-(4-aminophenyl)-propane, and 4,4′-oxybisaniline. In a preferredembodiment, the diamine of the present disclosure is at least onediaminoalkane selected from the group consisting of ethylenediamine,1,2-diaminopropane, 1,3-diaminopropane, 1,4-diaminobutane,1,5-diaminopentane, and 1,6-diaminohexane. In a most preferredembodiment, the diaminoalkane of the present disclosure is1,4-diaminobutane.

In addition to the diamines and/or polyamines, additional monomers whichare reactive towards isocyanate groups may also be used, including chainextending and termination agents, such as monoamines (e.g. ammonia,C1-C18 alkylamines, arylamines, C1-C12 alkylarylamines), as well asaliphatic, cycloaliphatic, and aromatic mono-, di-, and poly-C1 to C18alcohols. In particular, if a diol and/or a polyol are used in additionto the diamine, a hybrid polymer comprising polyurea-urethanes may beformed according to the current method.

The polyurea of the present disclosure may have a wide molecular weightdistribution. In one embodiment, the polyurea of the present disclosurehas an average molecular weight of 1-100 kDa, preferably 2-80 kDa,preferably 5-60 kDa, preferably 10-40 kDa, preferably 15-35 kDa,preferably 20-30 kDa.

In a preferred embodiment, the polyurea may be prepared by the stepsinvolving mixing the aforementioned diisocyanate (e.g. toluene2,4-diisocyanate and toluene-2,6-diisocyanate) and diamine (e.g.1,4-diaminobutane) in a solvent to form a reaction mixture, and heatingthe reaction mixture thereby forming the polyurea. Suitable solventsincluding, but not limited to, acetonitrile, tetrahydrofuran (THF),dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethyl acetate, butylacetate, 1,3-dimethyl-2-imidazolidinone, and 1-methyl-2-pyrrolidone(NMP) may be used in preparing the polyurea. In a preferred embodiment,acetonitrile is used as the solvent. Other organic solvents that may beused in addition to or in lieu of acetonitrile include, withoutlimitation, butane, pentane, n-hexane, cyclohexane, n-octane, isooctane,petroleum ether, benzene, toluene, xylene, methylene chloride,chlorobenzene, diethyl ether, and acetone.

Prior to the mixing step, the diamine may be dissolved in the solvent toform a first mixture, and the diisocyanate is mixed with the firstmixture to form the reaction mixture. Alternatively, the aforementionedreagents (i.e. diamine and diisocyanate) may be dissolved in the solventseparately to form respective solutions, which are then mixed to formthe reaction mixture. The mixing may occur via stirring, shaking,swirling, sonicating, blending, or by otherwise agitating the reactionmixture. The mixing may be performed by employing a rotary shaker, amagnetic stirrer, a centrifugal mixer, or an overhead stirrer. Inanother embodiment, the reaction mixture is left to stand (i.e. notstirred). In a preferred embodiment, the reaction mixture is agitatedusing a magnetic stirrer or an overhead stirrer at a speed of 100-2,000rpm, 200-1,500 rpm, 300-1,000 rpm, 400-800 rpm, 500-700 rpm, or about600 rpm at a temperature of 40-95° C., 45-90° C., 50-85° C., 55-80° C.,60-75° C., or 65-70° C. for 0.5-12 hours, 1-10 hours, 2-8 hours, 3-7hours, or about 6 hours. An external heat source, such as a water bathor an oil bath, an oven, microwave, or a heating mantle, may be employedto heat the reaction mixture. In one or more embodiments, a molar ratioof the diamine to the diisocyanate is in the range of 1:1 to 4:1,preferably 1.2:1 to 3.5:1, more preferably 1.5:1 to 3:1, or about 2:1.However, in certain embodiments, the molar ratio of the diamine to thediisocyanate is less than 1:1 or greater than 4:1. In one embodiment,the diamine is present in the reaction mixture at a concentration of0.05-10 M, preferably 0.1-5 M, preferably 0.2-2 M, preferably 0.4-1 M,or about 0.5 M. In a related embodiment, the diisocyanate is present inthe reaction mixture at a concentration of 0.02-8 M, preferably 0.04-4M, preferably 0.08-2 M, preferably 0.1-1 M, or about 0.25 M. In apreferred embodiment, the polyurea is collected as a powder that may beseparated and washed in acetone and then dried. In one embodiment, thepowder may be dried under vacuum until a constant weight is achieved. Ina preferred embodiment, the method used herein has a polyurea productyield of at least 85%, preferably at least 90%, preferably at least 92%,preferably at least 94%, preferably at least 96%, preferably at least97%, preferably at least 98%, preferably at least 99%. The product yieldis calculated as (weight of polyurea obtained/combined weight of diamineand diisocyanate)×100%.

A particle is defined as a small object that behaves as a whole unitwith respect to its transport and properties. Before pre-dissolving in apolar aprotic solvent, the polyurea of the present disclosure in any ofits embodiments may be in the form of particles of the same shape ordifferent shapes, and of the same size or different sizes. An averagediameter (e.g., average particle diameter) of the particle, as usedherein, refers to the average linear distance measured from one point onthe particle through the center of the particle to a point directlyacross from it. As used herein, microparticles are particles having anaverage diameter between 0.1 and 1,000 μm in size. Nanoparticles areparticles having an average diameter between 1 and 100 nm in size. Inone embodiment, the polyurea used herein may be in the form ofmicroparticles having an average diameter in a range of 0.1-1,000 μm,1-750 μm, 5-600 μm, 10-500 μm, 25-400 μm, 50-300 μm, 75-200 μm, or100-150 μm. In another embodiment, the polyurea may be in the form ofnanoparticles having an average diameter in a range of 1-99 nm, 5-90 nm,10-80 nm, 20-70 nm, 30-60 nm, or 40-50 nm. In a preferred embodiment,the polyurea is in the form of microparticles.

The particles (e.g. nanoparticles, microparticles) of the polyurea maybe spherical, ellipsoidal, oblong, ovoidal, or some other rounded shape.In an alternative embodiment, the particles may be angular, rectangular,prismoidal, or some other angular shape. In a preferred embodiment, thepolyurea particles are spherical. Microspheres are sphericalmicroparticles. In a more preferred embodiment, the polyurea is in theform of microspheres having a diameter of 1-1,000 μm, 5-900 μm, 10-800μm, 20-700 μm, 40-600 μm, 80-500 μm, 100-400 μm, or 200-300 μm. The sizeand shape of particles may be analyzed by techniques such as dynamiclight scattering (DLS), scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), and/or atomic force microscopy (AFM).

In a preferred embodiment, the polyurea particles of the presentdisclosure are monodisperse, having a coefficient of variation orrelative standard deviation, expressed as a percentage and defined asthe ratio of the particle size standard deviation (σ) to the particlesize mean (μ) multiplied by 100 of less than 25%, preferably less than10%, preferably less than 8%, preferably less than 6%, preferably lessthan 5%, preferably less than 4%, preferably less than 3%, preferablyless than 2%. In a preferred embodiment, the polyurea particles usedherein are monodisperse having a particle size distribution ranging from80% of the average particle size to 120% of the average particle size,preferably 90-110%, preferably 95-105% of the average particle size.

The surface of the polyurea particles may be mesoporous or microporous.The term “microporous” refers to a surface having an average porediameter of less than 2 nm (20 Å), while the term “mesoporous” refers toa surface having an average pore diameter of 2-50 (20-500 Å) nm. In oneembodiment, the polyurea used herein in any of its embodiments ismesoporous and has a pore size of 20-500 Å, 50-400 Å, 100-300 Å, 120-250Å, 150-200 Å, or 170-190 Å. In a more preferred embodiment, the polyureais microporous and has a pore size of 5-19 Å, preferably 8-18 Å,preferably 10-17 Å, preferably 12-16 Å, preferably 14-15 A. In certainembodiments, the polyurea has porous structures with a pore size largerthan 500 Å (50 nm), for example, a pore size of 80 nm, 100 nm, 500 nm, 1μm, 10 μm, 50 μm, or 100 μm. Pore size may be determined by techniquesincluding, but not limited to, gas adsorption (e.g. N₂ adsorption),mercury intrusion porosimetry, and imaging techniques such as scanningelectron microscopy (SEM), and x-ray computed tomography (XRCT).

The method of the present disclosure involves inhibiting corrosion of ametallic substrate by mixing or introducing the formulation containingthe aforementioned polyurea with/into an aqueous corrosive medium incontact with the metallic substrate. Alternatively, the method of thepresent disclosure may be applied for inhibiting corrosion of metallicsubstrates when the aqueous corrosive medium (e.g. aqueous acidicmedium) is used for acid pickling of the metallic substrates, scaleremoval and oil well acidification of a system involving the metallicsubstrates. In certain embodiments, the method involves contacting andcoating the metallic substrate with the formulation prior to, during, orsubsequent to immersion in a corrosive medium. Preferably, theformulation will be contacted with, or continuously or intermittentlyapplied to, the surfaces of metallic substrates, preferably in a spaceor volume where continued contact between the metallic substrate and theformulation can be maintained or refreshed.

As used herein, the formulation of the present disclosure includes atleast the polyurea and the polar aprotic solvent to pre-dissolve thepolyurea. In one or more embodiments, the polyurea is pre-dissolved in asuitable solvent prior to the introducing step. Polyurea is generallyknown to be insoluble in an aqueous medium. The pre-dissolving step isapplied herein to enhance the dispersion and solubility of the polyureain the aqueous corrosive medium.

Preferably, the solvent used to pre-dissolve the polyurea is at leastpartially soluble in water and preferably miscible with water. Morepreferably, the solvent used is a polar aprotic solvent. Otherconsiderations in the selection of solvents include low toxicity, lowcorrosive activity, low environmental hazard potential, availability,and cost. Exemplary polar aprotic solvents include, but are not limitedto, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO), 1,3-dimethyl-2-imidazolidinone, acetone,tetrahydrofuran (THF), ethyl acetate, and acetonitrile. In a preferredembodiment, the polar aprotic solvent used herein to pre-dissolve thepolyurea is at least one selected from the group consisting ofN-methyl-2-pyrrolidone, dimethylformamide, dimethyl sulfoxide, and1,3-dimethyl-2-imidazolidinone. In a most preferred embodiment, thepolar aprotic solvent used herein is N-methyl-2-pyrrolidone. It isequally envisaged that the present method may be adapted to incorporateother organic solvents such as methanol, ethanol, n-propanol,iso-propanol, n-butanol, 1,3-butanediol, 1,4-butanediol, 1,4-dioxane,furfuryl alcohol, and mixtures thereof. The pre-dissolving step may beperformed via agitation at an optionally elevated temperature. Methodsof agitation include, without limitation, swirling by hand, stirringwith a magnetic stir plate or a mechanical stirrer, shaking with arotary shaker, using a disperser (e.g. a high shear disperser), andsonicating using an ultrasonic bath or an ultrasonic probe. In oneembodiment, a weight ratio of the polyurea to the solvent pre-dissolvingthe polyurea is in a range of 1:2,500 to 1:2, preferably 1:2,000 to 1:5,preferably 1:1,000 to 1:10, preferably 1:800 to 1:25, preferably 1:600to 1:50, preferably 1:400 to 1:60, preferably 1:200 to 1:80, preferably1:150 to 1:90, or about 1:100.

In one or more embodiments, a volume ratio of the polar aprotic solvent(e.g. NMP) to the aqueous corrosive medium is in a range of 1:80 to1:1,000, preferably 1:82 to 1:800, preferably 1:84 to 1:600, preferably1:86 to 1:500, preferably 1:88 to 1:400, preferably 1:90 to 1:300,preferably 1:92 to 1:250, preferably 1:94 to 1:200, preferably 1:95 to1:150, preferably 1:96 to 1:125, preferably 1:97 to 1:120, preferably1:98 to 1:110, preferably 1:99 to 1:105, or about 1:100. However, incertain embodiments, the volume ratio of the polar aprotic solvent (e.g.NMP) to the aqueous corrosive medium is less than 1:1000 or greater than1:80.

As used herein, an epoxy ester resin is a polymer having epoxy estergroups (—CO—O—CH₂—CHOH—) formed by reacting epoxy groups with carboxylicacids. In one or more embodiments, the formulation is substantially freeof epoxy ester resin, for instance, the formulation comprises less than0.01 wt % of epoxy ester resin, preferably less than 0.005 wt %, morepreferably less than 0.001 wt % of epoxy ester resin, relative to atotal weight of the formulation. In at least one embodiment, theformulation is devoid of epoxy ester resin. The epoxy ester may be anunsaturated epoxy ester resin formed from the reaction of an aromaticdi-epoxide with an unsaturated C15 to C18 fatty acid. In at least oneembodiment, the formulation of the present method consists essentiallyof the polyurea and the polar aprotic solvent.

As used herein, a “corrosion inhibitor” refers to a chemical compound orformulation that when added to a material and/or contacted with acorrodible substrate, typically a metal or an alloy, decreases thecorrosion rates of the material. Inhibitors often play an important rolein the oil extraction and processing industries where they have alwaysbeen considered to be the first line of defense against corrosion. Theformulation of the current method (e.g. the pre-dissolved polyurea) mayinteract with the metallic substrate and form a cohesive and insolublefilm on the surface of the substrate. Organic inhibitor that containsheteroatoms (e.g. N, O, and S) may be adsorbed on a metallic surfacethus blocking the active corrosion sites. In a preferred embodiment, theformulation (e.g. the polyurea) is adsorbed onto the metallic substratethrough a chemisorption process which forms a protective film over thesurface of the substrate (see Examples 11 and 12, FIGS. 9, 10, 11A-D).In one embodiment, the protective film generated according to thecurrent method has a thickness of 1-500 μm, 5-250 μm, 10-125 μm, or25-50 μm.

As used herein, “parts per million” or “ppm” refers to an expression ofconcentration by mass or weight. For example, 1 ppm of a formulationdenotes a 1:1,000,000 weight ratio of formulation per total weight offluid (e.g. liquids, gases or combinations thereof) contacting themetallic substrate. Alternatively, 1 ppm of a formulation denotes a1:1,000,000 weight ratio of the formulation per total weight ofcorrosive fluid (e.g. acid pickling solutions, scale removal reagents)contained or carried within an infrastructure having the metallicsubstrates.

In one or more embodiments, the method disclosed herein involvesintroducing the polyurea into the aqueous corrosive medium at aconcentration of 5-500 ppm, 10-400 ppm, 20-300 ppm, 30-250 ppm, 40-200ppm, 50-150 ppm, 60-125 ppm, 70-100 ppm, or 80-90 ppm. However, incertain embodiments, the polyurea may be introduced into the aqueouscorrosive medium at a concentration less than 5 ppm or greater than 500ppm.

The method disclosed herein in any its embodiments may be effective inprotecting the aforementioned metallic substrate against corrosion invarious environments such as acidic and/or high salt concentrationenvironments. In one or more embodiments, the corrosive medium containsat least one acid. The acid may be acidic reagents commonly used foracid pickling, scale removal, well acidification, and acid fracturingprocesses. The acid may be in liquid or gas forms and include, but arenot limited to, hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitricacid (HNO₃), hydrofluoric acid (HF), acetic acid (CH₃COOH), formic acid(HCOOH), citric acid, phosphoric acid, carbon dioxide (CO₂), andhydrogen sulfide (H₂S). These acids may be present in the aqueouscorrosive medium at saturated concentrations, or at a concentration in arange of 0.01-10 M, 0.05-8 M, 0.1-6 M, 0.25-4 M, 0.5-3 M, 0.75-2 M, orabout 1 M. In at least one embodiment, the aqueous corrosive mediumcontains hydrochloric acid. In a preferred embodiment, the aqueouscorrosive medium has a pH of 5 or below, for example, a pH in a range of1-5, 1.5-4.5, 2-4, 2.5-3.5, or 2-3.

Inorganic salts such as sodium chloride have been known to cause seriouscorrosion to steels. As used herein, brine is an aqueous mixture of oneor more soluble salts (e.g. sodium chloride, potassium chloride, calciumchloride, calcium bromide, sodium bromide, potassium bromide, zincbromide, magnesium chloride). Seawater or water from a salt lake may beconsidered a brine. In some embodiments, brine may be present in theaqueous corrosive medium. For example, the aqueous corrosive medium maycontain 1-10 wt %, 2-5 wt %, or about 3.5 wt % sodium chloride, 0.1-1 wt%, 0.2-0.5 wt %, or about 0.3 wt % calcium chloride, as well as 0.05-1wt %, 0.1-0.4 wt %, or about 0.2 wt % magnesium chloride, each relativeto a total weight of the aqueous corrosive medium. In certainembodiments, the aqueous corrosive medium contains the acid and brine.

Miscibility can be estimated by using solubility parameters (δ), whichare tabulated for many different polymers and solvents. The Hildebrandsolubility parameter provides a numerical estimate of the degree ofinteraction between materials, and can be a good indication ofsolubility, particularly for materials such as polymers. Materials withsimilar values of δ are likely to be miscible. In a preferredembodiment, the formulation of the present disclosure (i.e. the polyureapre-dissolved in the polar aprotic solvent) is soluble in water (δ=23.4(cal/cm³)^(1/2)). In a preferred embodiment, the formulation of thepresent disclosure has a Hildebrand solubility parameter of 5-25(cal/cm³)^(1/2), preferably 8-22 (cal/cm³)^(1/2), more preferably 10-20(cal/cm³)^(1/2). In a preferred embodiment, the formulation (i.e. thepolyurea pre-dissolved in the polar aprotic solvent) is soluble inwater. In another preferred embodiment, the formulation is soluble in anacidic aqueous medium with a pH of up to 6.5, for example a pH in arange of 1-6.5, 1.5-6, 2-5.5, 2.5-5, 3-4.5, or 3.5-4. In a preferredembodiment, the formulation is soluble in the aqueous corrosive mediumdescribed previously.

Preferably, for applications in the oil and gas industry, theformulation of the present invention is stable at high temperatures. Oiland gas wells can reach temperatures higher than 120° C. (e.g. 130-140°C.). In one or more embodiments, the aqueous corrosive medium has atemperature in a range of 4-120° C., 10-100° C., 20-80° C., 30-70° C.,40-60° C., or 45-55° C.

As used herein, “corrosion” refers to the process which converts refinedmetals to their more stable oxide. It is the gradual loss of a material(usually metals) by chemical reaction with their environment. Commonly,this means electrochemical oxidation of metal in reaction with anoxidant such as oxygen. Rusting, the formation of iron oxides is awell-known example of electrochemical corrosion producing oxide(s)and/or salt(s) of the original metal. Corrosion degrades the usefulproperties of materials and structures including strength, appearanceand permeability to liquids and gases. Many structural alloys corrodemerely from exposure to moisture in air, but the process can be stronglyaffected by exposure to certain substances. Because corrosion is adiffusion-controlled process, it generally occurs on exposed surfaces.

Exemplary metallic substrates applicable to the present disclosureinclude, but are not limited to, copper, copper alloys (e.g. brass orbronze), aluminum, aluminum alloys (e.g. aluminum-magnesium,nickel-aluminum, aluminum-silicon), nickel, nickel alloys (e.g.nickel-titanium or nickel-chromium), iron, iron alloys, carbon steels,alloy steels, stainless steels, and tool steels.

Steel is an alloy of iron and carbon that is widely used in constructionand other applications because of its high tensile strength and lowcost. Carbon, other elements, and inclusions within iron act ashardening agents that prevent the movement of dislocations thatnaturally exist in the iron atom crystal lattices. The carbon in typicalsteel alloys may contribute up to 2.1% of its weight.

Steels can be broadly categorized into four groups based on theirchemical compositions: carbon steels, alloy steels, stainless steels,and tool steels. Carbon steels contain trace amounts of alloyingelements and account for 90% of total steel production. Carbon steelscan be further categorized intro three groups depending on their carboncontent: low carbon steels/mild steels contain up to 0.3% carbon, mediumcarbon steels contain 0.3-0.6% carbon, and high carbon steels containmore than 0.6% carbon. Alloys steels contain alloying elements (e.g.manganese, silicon, nickel, titanium, copper, chromium and aluminum) invarying proportions in order to manipulate the steel's properties, suchas its hardenability, corrosion resistance, strength, formability,weldability or ductility. Stainless steels generally contain between10-20% chromium as the main alloying element and are valued for highcorrosion resistance. With over 11% chromium, steel is about 200 timesmore resistant to corrosion than mild steel. These steels can be dividedinto three groups based on their crystalline structure: austeniticsteels, ferritic steels and martensitic steels. Tool steels containtungsten, molybdenum, cobalt and vanadium in varying quantities toincrease heat resistance and durability, making them ideal for cuttingand drilling equipment.

In one embodiment, the metallic substrate comprises steel, carbon steel,low carbon steel, mild steel, medium carbon steel, high carbon steel,alloy steel, stainless steel, austenitic steel, ferritic steel,martensitic steel, tool steel, or mixtures thereof. Preferably, themetallic substrate comprises carbon steel. More preferably, the metallicsubstrate is a mild steel with a carbon content of up to 0.3%,preferably a carbon content of 0.1-0.25%, preferably a carbon content of0.15-0.2%, for example, 1018 (e.g. AISI 1018), ASTM A36, 12L14, ASTMA653, and other steel alloys such as A366/1008, A513 (alloy 1020-1026),8620 alloy.

As used herein, “systems” include, but are not limited to, systems usedin petroleum (e.g., crude oil and its products) or natural gasproduction, such as well casing, transport pipelines, drilling and otheroil field applications, transport, separation, refining, storage, andother liquid natural gas and petroleum-related applications, geothermalwells, water wells; cooling water systems including open recirculating,closed, and once-through systems; cisterns and water collection orholding systems, solar water heating systems, boilers and boiler watersystems or systems used in power generation, mineral process watersincluding mineral washing, flotation and benefaction; paper milldigesters, washers, bleach plants, white water systems and mill watersystems; black liquor evaporators in the pulp industry; gas scrubbersand air washers; continuous casting processes in the metallurgicalindustry; air conditioning and refrigeration systems; building fireprotection heating water, such as pasteurization water; waterreclamation and purification systems; membrane filtration water systems;food processing streams and waste treatment systems as well as inclarifiers, liquid-solid applications, municipal sewage treatmentsystems; and industrial or municipal water distribution systems.

In preferred embodiments, the metallic substrate is part of a system foroil or gas production, transportation, or refining. The metallicsubstrate may be part of a system used in the drilling, petroleum, oiland gas industries including drills, drill bits, pumps, compressors,pipelines, and other tools and equipment, electric parts such astransformers, power generators and electric motors, vehicle partsincluding those of boats, autos, trucks, aircraft, and militaryvehicles. Tools, including construction, automotive, household, andkitchen tools, are included.

Corrosion rate is the speed at which metals undergo deterioration withina particular environment. The rate may depend on environmentalconditions and the condition or type of metal. Factors often used tocalculate or determine corrosion rate include, but are not limited to,weight loss (reduction in weight during reference time), area (initialsurface area), time (length of reference time) and density. Corrosionrate is typically computed using millimeter penetration per year (mmpy)or mils per year (mpy). Mils penetration per year (mpy) is a unit ofmeasurement equal to approximately one thousandth of an inch. In metricexpression 1 mil is equal to 0.0254 mm, accordingly, 1 mpy is equal to0.0254 mmpy.

In one or more embodiments, the method of the present disclosure in anyof its embodiments imparts a corrosion rate in a range of 0.005-1.1millimeter penetration per year (mmpy) to the metallic substrate,preferably 0.01-1 mmpy, preferably 0.02-0.5 mmpy, preferably 0.04-0.4mmpy, preferably 0.05-0.3 mmpy, preferably 0.075-0.2 mmpy, preferably0.1-0.18 mmpy, preferably 0.12-0.15 millimeter penetration per year(mmpy). In a preferred embodiment, the corrosion rate of the metallicsubstrate according to the presently disclosed method may be slowed by2-100 mmpy relative to the corrosion rate of a substantially identicalmetallic substrate exposed to a substantially identical aqueouscorrosive medium lacking the formulation, preferably slowed by 10-96mmpy, 20-90 mmpy, 30-80 mmpy, 40-70 mmpy, or 50-60 mmpy relative to thecorrosion rate of a substantially identical metallic substrate exposedto a substantially identical aqueous corrosive medium lacking theformulation (see Table 2).

Corrosion inhibition efficiencies may be measured with the Tafelextrapolation, linear polarization resistance (LPR), potentiodynamicpolarization (PDP), gravimetric or other similar methods. In a preferredembodiment, the method described herein in any of its embodimentsachieves a corrosion inhibition efficiency in a range of 70-100%,75-99.9%, 80-99%, 85-98%, 90-97%, 92-96%, or 94-95%.

In a preferred embodiment, the method disclosed herein has a corrosioninhibition efficiency in a range of 70-100% when the polyurea of theformulation is introduced to the aqueous corrosive medium at aconcentration ranging from 1-200 ppm, preferably a corrosion inhibitionefficiency greater than 95% at a concentration of the polyurea of atleast 100 ppm, preferably greater than 95% at a concentration of thepolyurea of up to 75 ppm, preferably greater than 95% at a concentrationof the polyurea of up to 50 ppm, preferably greater than 95% at aconcentration of the polyurea of up to 40 ppm, preferably greater than95% at a concentration of the polyurea of up to 30 ppm, preferablygreater than 95% at a concentration of the polyurea of up to 25 ppm,preferably greater than 95% at a concentration of the polyurea of up to20 ppm (see Tables 2 and 3).

The examples below are intended to further illustrate protocols forpreparing, characterizing the polyurea and the formulation containingthe polyurea pre-dissolved in the polar aprotic solvent, and usesthereof, and are not intended to limit the scope of the claims.

Example 1

Reagents

All chemicals utilized in the current disclosure were of high purity andused without further purifications. Toluene diisocyanate (TDI, a mixtureof 2,4-(80%) and 2,6-(20%)-isomers) was purchased from Fluka Chemika(Germany). 1,4-diaminobutane (DAB) from Alfa Aesar (Germany),acetonitrile from Merck (Germany), hydrochloric acid from FisherScientific (USA) and N-methyl-2-pyrrolidone from Riedel—de Haen(Germany). 1.0 M HCl solution used for corrosion tests was prepared bydilution using double distilled water.

Example 2

Synthesis of Polyurea (PU)

An established procedure was utilized in the synthesis of PU withmodifications (FIG. 1 ) [F. Zhang, X. Jiang, X. Zhu, Z. Chen, X. Z.Kong, Preparation of uniform and porous polyurea microspheres of largesize through interfacial polymerization of toluene diisocyanate in watersolution of ethylene diamine, Chemical Engineering Journal, 303 (2016)48-55, incorporated herein by reference in its entirety]. Diaminobutane,DAB (0.88 g, 10 mmol) was added to 20 mL acetonitrile in a three-neckedround bottomed flask and immersed in an oil bath at 60° C. until it wasdissolved completely. Thereafter, toluene diisocyanate, TDI (0.90 g, 5mmol) was added drop wise to DAB while stirring at 600 rpm, and thepolymerization was allowed to continue for 6 h. At the end of thereaction, a whitish product was formed which was filtered, washed inacetone, and dried up in a vacuum oven at 70° C. until a constant masswas reached. The resulting powder weighed 1.70 g (96% yield). FTIR; 3350cm⁻¹ (s), 3000 cm⁻¹ (m), 1650 cm⁻¹ (s), 1550 cm⁻¹ (s), 1230 cm⁻¹ (s),700 cm⁻¹ (w).

Example 3

Characterization of PU

¹H and ¹³C NMR spectra of the synthesized PU in DMSO-d₆ were recorded ona JEOL 500 MHz NMR spectrometer using TMS as internal standard. IRspectrum was recorded on a Perkin Elmer 16F PC FTIR spectrometer usingKBr as standard. Thermogravimetric analysis (TGA) was done on an SDTanalyzer (Q600: TA instruments, USA). Approximately 5.8 mg of thepolymer was taken in an aluminum crucible. The temperature was raised ata controlled rate of 15° C./min. The analysis was made over atemperature range of 20-1000° C. in air. Morphological studies of thesteel specimen after exposure to 1.0 M HCl in the absence and presenceof PU was carried out using scanning electron microscope, SEM(Genesis-2120 Emcrafts, Korea) and x-ray photoelectron spectrometer, XPS(Thermo Scientific ESCALAB 250 Xi).

Example 4

Electrochemical Measurements

Electrochemical measurements were performed in a jacketedelectrochemical cell kit with three-electrode compartments (Gamry). Mildsteel bar AISI 1018 with the following compositions; C, 0.15-0.20%; Mn,0.60-0.90%; P, 0.04%; S, 0.05% and balance Fe was used as the samplespecimen, saturated calomel electrode (SCE) and carbon graphite as thereference and counter electrodes, respectively. The sample specimen waspolished sequentially using silicon carbide emery papers of differentgrit size (120-800), rinsed with distilled water, placed in anultrasonic acetone bath to remove possible residue due to polishing,rinsed with acetone, dried in warm air, and stored in a moisture-freedesiccator before use.

Gamry potentiostat/galvanostat/ZRA (Reference 600) equipped with a Gamryframework interface was used for electrochemical studies. Gamry EchemAnalyst version 6.03 was used for data analysis and fittings. Opencircuit potential measurements were carried out during two hours ofimmersion in freshly prepared polymer solutions.

Polarization (Tafel) curves were obtained by scanning the potential at0.5 mV/s in the range +250 mV to −250 mV against the open circuitpotential (E_(ocp)). Corrosion potential (E_(corr)/V vs SCE) andcorrosion current density (i_(corr), A cm⁻²) were obtained byextrapolation of anodic (βa) and cathodic (βc) branches of the Tafelcurves.

Electrochemical impedance spectroscopy measurements (EIS) were carriedout at open circuit potential (E_(ocp) vs SCE) in the frequency range100 KHz to 100 mHz, with an amplitude of perturbation 10 mV r.m.s.

Example 5

DFT Calculations

DFT calculations were carried out using the Gaussian 09 package [M. J.Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H.Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J.Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R.Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M.Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R.Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E.Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Farkas, J. B.Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, RevisionB.01, in, Wallingford CT, 2009], with hybrid function of the Beckethree-parameters Lee, Yang and Par (B3LYP) method and the 6-31G(d) basisset. Structural geometry of each molecule was tested and the vibrationalfrequency analysis shows no imaginary frequencies. Structural featuressuch as global hardness (η), electron affinity (A), ionization potential(I), electronegativity (χ), and dipole moment (μ) were subsequentlycalculated.

Example 6

Results and Discussions: Characterization of PU

NMR spectroscopy was utilized to investigate the formation and purity ofpolyurea synthesized in the present study. In order to better understandthe results, the monomers, 1,4-diaminobutane (DAB) and TDI (a mixture of2,4- and 2,6-isomers) were also analyzed using ¹³C and ¹H NMRspectroscopies.

The ¹³C NMR spectra of both monomers and PU polymer are presented inFIGS. 2A and 2B. As shown in FIG. 2A, 1,4-diaminobutane has tworesonance peaks in the ¹³C NMR which were both found with chemicalshifts at 30.98 ppm and 41.76 ppm corresponding to the carbon atomslabeled as 1″ and 2″, respectively. TDI consists of two isomers (2,4-and 2,6-TDI), hence multiple resonance peaks were observed. Assignmentof ¹³C NMR resonance peaks followed a reported procedure [H. Han, S. Li,X. Zhu, X. Jiang, X. Z. Kong, One step preparation of porous polyurea byreaction of toluene diisocyanate with water and its characterization,RSC Advances, 4 (2014) 33520-33529, incorporated herein by reference inits entirety] and peaks are labeled as shown in FIG. 2A. In the case ofpolyurea, it is important to note that resonance peaks were weak due tolimited solubility of polyurea in DMSO. However, with increased numberof scans and enlarged spectra (FIG. 2B), it can be seen that peaksassigned to carbon atoms attached to —NCO of TDI (C₂, C_(2′), C₄ andC_(6′)) at 132.55 ppm and 132.83 ppm all disappeared in PU spectrum.Furthermore, peaks assigned to —NCO carbons (C₈, C₉ and C_(8′)) at131.30 ppm and 124.20 ppm were also absent. New peaks appeared at around155.27 ppm, which have been assigned to characteristic resonances ofcarbonyl linkage (C═O) in polyurea [H. Han, S. Li, X. Zhu, X. Jiang, X.Z. Kong, One step preparation of porous polyurea by reaction of toluenediisocyanate with water and its characterization, RSC Advances, 4 (2014)33520-33529, incorporated herein by reference in its entirety]. Thisobservation confirms the formation of polyurea from the reactedmonomers.

Monomers and polymer were further characterized by ¹H NMR spectroscopyand the results are presented in FIGS. 3A and 3B. Notably from the ¹HNMR spectrum of DAB (FIG. 3A), a broad peak was observed at chemicalshift 1.45 ppm corresponding to the terminal amino hydrogen atoms. Thebroadening of the observed peak may be due to acidic nature of theseprotons that tend to exchange with one another. Likewise, two additionalpeaks were observed at 1.33 ppm and 2.49 ppm assigned to protons labeledas 2″ and 1″, respectively. The proton resonance at 2.49 ppm was notclearly visible as it overlapped with that of residual solvent (DMSO)signal at 2.50 ppm. In the case of TDI, resonance peaks of the methylgroups (H₇ and H_(7′)) appeared at 2.51 ppm and also overlapped with thesolvent signal. Aromatic protons which appeared in the range of chemicalshifts 6.0-8.0 ppm were also assigned as shown. For polyurea, however,multiple peaks appeared between chemical shifts 1.30 ppm and 4.0 ppm andhave been assigned to protons in DAB (1″ and 2″), DMSO signal, andmethyl protons (H₇ and H_(7′)) on TDI and residual water present in DMSOas shown in FIG. 3B. Furthermore, resonance peak of protons in the urea(—NH) linkage appeared at 6.00 ppm, while those for aromatic protonsgave a complicated splitting pattern in the range of 6.30-8.55 ppm aspreviously assigned [H. Han, S. Li, X. Zhu, X. Jiang, X. Z. Kong, Onestep preparation of porous polyurea by reaction of toluene diisocyanatewith water and its characterization, RSC Advances, 4 (2014) 33520-33529,incorporated herein by reference in its entirety] due to the presence oftwo isomers of TDI. All these confirmed the formation of polyurea fromthe reacted monomers TDI and DAB in the present disclosure.

FT-IR spectrum of PU (FIG. 4A) shows the presence of C═O stretch of theurea linkage at 1650 cm⁻¹, and a substituted —NH stretch at 3350 cm⁻¹[Y. Yang, X. Jiang, X. Zhu, X. Z. Kong, A facile pathway to polyureananofiber fabrication and polymer morphology control in copolymerizationof oxydianiline and toluene diisocyanate in acetone, RSC Advances, 5(2015) 7426-7432, incorporated herein by reference in its entirety]. Thestrong absorption band at 1550 cm⁻¹ represents the —NH bend while —CHstretch band appeared around 3000 cm⁻¹ [H. Han, S. Li, X. Zhu, X. Jiang,X. Z. Kong, One step preparation of porous polyurea by reaction oftoluene diisocyanate with water and its characterization, RSC Advances,4 (2014) 33520-33529, incorporated herein by reference in its entirety].Others are the —NH wag, —CH and —CN stretch bands at 700 cm⁻¹, 2930 cm⁻¹and 1230 cm⁻¹, respectively.

The TGA curve (FIG. 4B) shows 3 major losses in weight. The first slowweight loss of 15% is due to the loss of water embedded inside thepolyurea at approx. 120° C. The second and third losses at approx. 300°C. and 550° C. are due to the combustion of nitrogenous organicfractions present in the polymer with the release of CO₂, NOx, and H₂Ogases [T. A. Saleh, S. A. Haladu, S. A. Ali, A novel cross-linkedpH-responsive tetrapolymer: Synthesis, characterization and sorptionevaluation towards Cr(III), Chemical Engineering Journal, 269 (2015)9-19, incorporated herein by reference in its entirety].

Morphology of polyurea determined by scanning electron microscope (SEM)and elemental composition determined by EDX revealed a cluster ofspherical microporous materials with micro-sized particle sizecomprising mainly of carbon, nitrogen and oxygen. X-ray diffractionspectrometry (XRD) was used to check the crystallinity of the polymerand the result revealed a broad peak characteristic of amorphousmaterials which confirms the formation of PU in the present disclosure.

Example 7

Results and Discussions: DFT

A major breakthrough of the present disclosure is the fact that polyureawhich is generally known to be insoluble in an aqueous medium is beingutilized as a corrosion inhibitor in mild steel corrosion. In one aspectthis was done through solubilizing polyurea in N-methyl-2-pyrrolidonebefore dilution with the acid solution. In order to understand the rolethe solvent plays in the corrosion inhibitive performance of thepolymer, electronic calculations were carried out on both molecules.

FIG. 5 shows the structural geometry and frontier molecular orbitaldistribution of the molecules. In all cases electron density of thehighest occupied molecular orbital (HOMO) of the solvent and the polymerare localized mainly on the nitrogen and oxygen atoms present in bothmolecules and on the aromatic ring of the polymer. This indicates thefeasibility of bonding of the molecules to the vacant d-orbital of themetal. However, in the case of the solvent, resonation of thenon-bonding electrons between nitrogen and oxygen atoms makes theelectrons unavailable for bonding most of the time. Accordingly, therole played by the solvent in the adsorption of the polymer is quiteinsignificant. Furthermore, energy gap between the HOMO and LUMOorbitals (ΔE) indicates that the polymer having a lower energy gap ispredicted to be more reactive compared to the solvent.

Global hardness (η) is another feature which indicates the chemicalreactivity of a molecule. It indicates the resistance of a molecule toelectron density change. According to the hard-soft acid base theory(HSAB) [A. Kokalj, On the HSAB based estimate of charge transfer betweenadsorbates and metal surfaces, Chemical Physics, 393 (2012) 1-12],molecules with larger η are less reactive compared to molecules withsmaller η which are more polarizable and more prone to acquireelectronic charge. Results from electronic calculations are given inTable 1 which clearly shows that the role played by the solvent in thepresent study is simply that of a dispersion medium and makes little orno contribution to the observed inhibitive performance of the polymer.

TABLE 1 Electronic property of molecules Molecule E_(HOMO) (eV) E_(LUMO)(eV) ΔE_(L-H) (eV) η (eV) N-methyl-2-pyrrolidone −6.736 −0.443 6.2933.147 PU −6.033 −0.661 5.372 2.686

Example 8

Electrochemistry Results: Open Circuit Vs Time Measurement

Open circuit potential (OCP) is the potential measured when no externalcurrent is passed through the metal in the corrosive medium. It is athermodynamic parameter that indicates the tendency of a metal toundergo electrochemical corrosion. When immersed in the electrolyte themetal begins to corrode naturally without an application of currentuntil it reaches a stationary state. At this point open circuitpotential (Eocp) is equal to the corrosion potential (Ecorr). Hence apotential below Eocp is said to be thermodynamically stable and a metalhaving such potential is more susceptible to undergo corrosion. OCPmeasurements generally provide information whether the metal under studyis in the active or passive state and serves as a starting point for theapplication of electrochemical methods of corrosion measurement [S.Choudhary, A. Garg, K. Mondal, Relation Between Open Circuit Potentialand Polarization Resistance with Rust and Corrosion Monitoring of MildSteel, Journal of Materials Engineering and Performance, 25 (2016)2969-2976; S. Saker, N. Aliouane, H. Hammache, S. Chafaa, G. Bouet,Tetraphosphonic acid as eco-friendly corrosion inhibitor on carbon steelin 3% NaCl aqueous solution, Ionics, 21 (2015) 2079-2090; and I.Felhősi, J. Telegdi, G. Pálinkás, E. Kálmán, Kinetics of self-assembledlayer formation on iron, Electrochimica Acta, 47 (2002) 2335-2340].

The Eocp curves of mild steel in 1.0 M HCl in the presence and absenceof varying concentrations of PU (20, 50 and 100 ppm) are shown in FIG. 6. As shown in the Figure, a steady state was reached nearly after 300 sof immersion in the presence of the inhibitor molecules while in itsabsence the metal continues to corrode even after 7200 s. The blank inthis case refers to a solution containing 1.0 M HCl andN-methyl-2-pyrrolidone which is used as a solvent for dissolving thepolymer in the ratio 100:1. This confirms the findings from DFTcalculations and shows that the presence of the solvent plays little orno role in the inhibitive performance observed of the polymer.Furthermore, the Eocp of the inhibitor molecules shifts to more noblevalues with increasing concentration which indicates an increase insurface coverage on the metal consequently leading to a decrease incorrosion rate.

Example 9

Electrochemistry Results: Potentiodynamic Polarization Measurements

Potentiodynamic polarization (Tafel) measurements provide information onthe kinetics of partial anodic and cathodic reactions. In thistechnique, the potential of the working electrode (metal) is scanned inthe positive (anodic) and negative (cathodic) directions and informationsuch as corrosion rate, pitting susceptibility and passivity of themetal can be extracted from the resulting Tafel curve. In order to fullyunderstand the mechanism of inhibition of mild steel corrosion bypolyurea in the present study, it is important to highlight themechanism through which corrosion of iron takes place.

First, upon contact with the corrosive medium (HCl in this case), Feundergoes oxidation to form Fe²⁺ as follows [A. Zeino, I. Abdulazeez, M.Khaled, M. W. Jawich, I. B. Obot, Mechanistic study of polyaspartic acid(PASP) as eco-friendly corrosion inhibitor on mild steel in 3% NaClaerated solution, Journal of Molecular Liquids, 250 (2018) 50-62]Fe

Fe²⁺+2e ⁻  (1)The Fe²⁺ generated in the process combines with the chloride ions in themedium and later with water molecules and oxygen atoms in a series ofreactions as follows;Fe²⁺+2Cl⁻

FeCl₂  (2)FeCl₂+2H₂O

Fe(OH)₂+2H⁺+2Cl⁻  (3)Fe²⁺+O₂

Fe³⁺ +e ⁻  (4)The cathodic reaction which completes the electrochemical process isrepresented as follows;O₂+2H₂O+4e ⁻

4OH⁻  (5)

From the combined anodic and cathodic reactions, precipitates of Fe(OH)₂are formed which gradually accumulate on the metal surface and furtherreact with dissolved oxygen giving complex corrosion products [S. Saker,N. Aliouane, H. Hammache, S. Chafaa, G. Bouet, Tetraphosphonic acid aseco-friendly corrosion inhibitor on carbon steel in 3% NaCl aqueoussolution, Ionics, 21 (2015) 2079-2090, incorporated herein by referencein its entirety].

Organic inhibitors are believed to adsorb on iron by first displacingwater molecules on the metal surface followed by combination with Fe²⁺generated on the surface to form Fe-inhibitor complex [H.Ashassi-Sorkhabi, E. Asghari, Effect of hydrodynamic conditions on theinhibition performance of 1-methionine as a “green” inhibitor,Electrochimica Acta, 54 (2008) 162-167, incorporated herein by referencein its entirety] as follows;Inh._((aq)) +xH₂O_((ads))

Inh._((ads)) +xH₂O_((aq))  (6)Fe²⁺Inh._((ads))

[Fe−Inh.]_((ads)) ²⁺  (7)

The complex formed either prevents more water molecules from reachingthe metal (inhibitor) or enhances the permeation of water molecules(accelerator). If the concentration is high enough up to the criticalmass of the inhibitor a complete coverage of the metal's surface isachieved preventing the diffusion of oxygen atoms to complete thecathodic half reaction, forcing the metal into passivation. On the basisof these, inhibitors are classified as either anodic, cathodic ormixed-type corrosion inhibitors [H. Ju, Z.-P. Kai, Y. Li, Aminicnitrogen-bearing polydentate Schiff base compounds as corrosioninhibitors for iron in acidic media: A quantum chemical calculation,Corrosion Science, 50 (2008) 865-871].

Potentiodynamic polarization measurement curves carried out on mildsteel at various temperatures and inhibitor concentrations are presentedin FIGS. 7A-7E. The corresponding Tafel extrapolated data are listed inTable 2.

TABLE 2 Tafel polarization data of mild steel at varying temperaturesand concentrations Inhibitor E_(corr)/ Corrosion Temperature Conc. V vsI_(corr) rate (K) Medium (ppm) SCE (μA · cm⁻²) (mmpy) θ η % 298 Blank 0−0.512 301.1 3.43 — — PU 20 −0.420 87.0 1.02 0.703 70.3 50 −0.420 15.20.18 0.948 94.8 100 −0.402 5.0 0.06 0.983 98.3 303 Blank 0 −0.539 470.25.46 — — PU 20 −0.494 28.1 0.33 0.939 94.0 50 −0.417 17.0 0.20 0.96396.3 100 −0.416 1.2 0.02 0.996 99.6 308 Blank 0 −0.542 3070.3 35.56 — —PU 20 −0.471 2.0 0.02 0.999 100 50 −0.561 0.1 0.01 0.999 100 100 −0.5820.1 0.01 0.999 100 313 Blank 0 −0.521 4400.5 51.08 — — PU 20 −0.479 31.30.36 0.994 99.4 50 −0.534 11.1 0.13 0.998 99.8 100 −0.526 3.6 0.03 0.999100 323 Blank 0 −0.504 8430.2 96.5 — — PU 20 −0.539 2.2 0.02 0.999 10050 −0.551 0.3 0.01 0.999 100 100 Passivation — — — —

Corrosion inhibition efficiency was calculated using the equation:

$\begin{matrix}{{\eta(\%)} = {\left( \frac{i_{corr}^{\circ} - i_{corr}}{i_{corr}^{\circ}} \right) \times 100}} & (8)\end{matrix}$where i°_(corr) and i_(corr) are corrosion current density values in theabsence and presence of the inhibitors respectively [E. Gutiérrez, J. A.Rodríguez, J. Cruz-Borbolla, J. G. Alvarado-Rodríguez, P. Thangarasu,Development of a predictive model for corrosion inhibition of carbonsteel by imidazole and benzimidazole derivatives, Corrosion Science, 108(2016) 23-35, incorporated herein by reference in its entirety].

The results showed that addition of the inhibitor significantlydecreases the corrosion current (i_(corr)) which agrees with the resultsof the open circuit vs time measurement. Corrosion current continuouslydecreases upon increasing the concentration of the inhibitor due to anincrease in surface coverage of the inhibitor on the metal surface. Thecorrosion potential (E_(corr)) on the other hand is observed to havemixed behaviors, where it shifts to positive in some cases and negativein others, an attribute of mixed-type corrosion inhibitors. As thetemperature increases, the corrosion rate in the blank solutionincreases while the efficiency of the inhibited medium also increases,and the result at 50° C. and 100 ppm inhibitor concentration suggests apassivation control corrosion mechanism. This is because at higherconcentrations and temperature, there is less dissolved oxygen in themedium and the inhibitor spreads out to achieve maximum surface coverageshifting the Ecorr into the passivation region as was later confirmed byspectroscopic studies. Overall, a 100% corrosion inhibition with acorrosion current less than 1 μA was achieved at 50 ppm and 50° C.making polyurea synthesized in the present disclosure an excellentinhibitor of mild steel corrosion in aqueous environment [B. D. B. Tiu,R. C. Advincula, Polymeric corrosion inhibitors for the oil and gasindustry: Design principles and mechanism, Reactive and FunctionalPolymers, 95 (2015) 25-45; S. Kumar, H. Vashisht, L. O. Olasunkanmi, I.Bahadur, H. Verma, G. Singh, I. B. Obot, E. E. Ebenso, Experimental andtheoretical studies on inhibition of mild steel corrosion by somesynthesized polyurethane tri-block co-polymers, Scientific Reports, 6(2016) 30937; and M. Goyal, S. Kumar, I. Bahadur, C. Verma, E. E.Ebenso, Organic corrosion inhibitors for industrial cleaning of ferrousand non-ferrous metals in acidic solutions: A review, Journal ofMolecular Liquids, 256 (2018) 565-573, each incorporated herein byreference in their entirety].

Example 10

Electrochemistry Results: Electrochemical Impedance SpectroscopyMeasurements

EIS measurements were carried out to understand the kinetics andcharacteristics of the electrochemical processes occurring at thesteel/solution interface in the presence of the inhibitor molecule.FIGS. 8A-8F show resulting Nyquist (a and b) and Bode (c and d) plots ofmild steel corrosion in 1.0 M HCl in the absence and presence of 20-100ppm inhibitor at 25° C. and 40° C. Nyquist plots (FIGS. 8A, 8B, 8C, and8D) are characterized by single depressed semi-circles which indicatethat dominating process in the studied medium is the charge transferreaction occurring at the metal/solution interface [C. Verma, L. O.Olasunkanmi, E. E. Ebenso, M. A. Quraishi, I. B. Obot, AdsorptionBehavior of Glucosamine-Based, Pyrimidine-Fused Heterocycles as GreenCorrosion Inhibitors for Mild Steel: Experimental and TheoreticalStudies, The Journal of Physical Chemistry C, 120 (2016) 11598-11611,incorporated herein by reference in its entirety]. In addition, a singlecapacitive loop was present which implies that the studied inhibitoracts primarily as interface inhibitor through surface coverage mechanism[R. Solmaz, Investigation of corrosion inhibition mechanism andstability of Vitamin B1 on mild steel in 0.5M HCl solution, CorrosionScience, 81 (2014) 75-84, incorporated herein by reference in itsentirety]. Diameter of the semi-circle also increases with increasinginhibitor concentration due to the increase in surface coverage by theinhibitor molecules. Bode plots (FIGS. 8E and 8F) also show the effectof the presence of the inhibitor molecules at the lower frequencymodulus as the low frequency impedance increases with increasinginhibitor concentration. This is due to the adsorption of inhibitormolecules onto exposed steel surface thereby blocking the activecorrosion sites [Z. Cao, Y. Tang, H. Cang, J. Xu, G. Lu, W. Jing, Novelbenzimidazole derivatives as corrosion inhibitors of mild steel in theacidic media. Part II: Theoretical studies, Corrosion Science, 83 (2014)292-298, incorporated herein by reference in its entirety].

Inhibition efficiency of the inhibitor molecules were calculated usingthe equation:

$\begin{matrix}{{\eta(\%)} = {\left( \frac{R_{ct} - R_{ct}^{\circ}}{R_{ct}} \right) \times 100}} & (9)\end{matrix}$where R_(ct) and R°_(ct) are the charge transfer resistances in thepresence and absence of the inhibitor molecules, respectively [E.Gutiérrez, J. A. Rodriguez, J. Cruz-Borbolla, J. G. Alvarado-Rodríguez,P. Thangarasu, Development of a predictive model for corrosioninhibition of carbon steel by imidazole and benzimidazole derivatives,Corrosion Science, 108 (2016) 23-35].

Equivalent circuit model used for fitting and obtaining EIS datacomprises of a constant phase element, CPE in parallel with a chargetransfer resistance, Rct both of which are in series connection with theelectrolyte resistance, Rs. Impedance of the constant phase element,Z_(CPE) is given as:Z _(CPE) =Y _(O) ⁻¹(iω)^(−n)  (10)where Yo represents CPE constant, i² an imaginary number (−1), ω angularfrequency (rad s⁻¹) and n is CPE exponent. Values of n characterizesCPE, where n=0 implies that CPE=resistance; n=1, CPE=capacitance; n=−1,CPE=inductance and n=0.5, CPE=Warburg impedance [C. Verma, L. O.Olasunkanmi, T. W. Quadri, E.-S. M. Sherif, E. E. Ebenso, Gravimetric,Electrochemical, Surface Morphology, DFT, and Monte Carlo SimulationStudies on Three N-Substituted 2-Aminopyridine Derivatives as CorrosionInhibitors of Mild Steel in Acidic Medium, The Journal of PhysicalChemistry C, 122 (2018) 11870-11882]. Double layer capacitance, C_(dl)was estimated using the equation:

$\begin{matrix}{C_{dl} = \frac{1}{2\pi f_{\max}R_{ct}}} & (11)\end{matrix}$where f_(max) represents the maximum frequency of the imaginarycomponent of the impedance and R_(ct) the charge-transfer resistance.

In the absence of the inhibitor, mild steel corroded freely in acidicmedium without a resistive barrier giving rise to large double layercapacitance, C_(dl) and lower charge transfer resistance, R_(ct) (Table3) and this was even more pronounced at higher temperature. However,when inhibitor was introduced, there was a significant drop in thevalues of the C_(dl) and a corresponding increase in R_(ct). This isbecause in the presence of the inhibitor molecules, charge transferprocesses leading to the corrosion of the metal was prevented byadsorbed films of the inhibitor molecules thereby slowing down thecorrosion rate. An overall inhibition efficiency of 98.9% was achievedat 40° C. and 100 ppm inhibitor molecule as shown in Table 3.

TABLE 3 EIS data of mild steel corrosion in 1.0M HCl with inhibitormolecules Inhibitor Temperature conc. Rs Rct Cdl (K) Medium (ppm) (Ω ·cm²) (Ω · cm²) (μF · cm⁻²) n η % 298 Blank 0 1.237 29.54 431.0 0.91 — PU20 2.165 160.2 31.5 0.84 81.6 50 2.120 313.4 25.7 0.82 90.6 100 2.104402.3 25.1 0.82 93.0 313 Blank 0 1.00 15.56 1296.3 0.88 — PU 20 5.83722.4 14.0 0.85 97.9 50 9.86 1163.0 13.7 0.85 98.7 100 10.76 1430.0 11.10.82 98.9

Example 11

Thermodynamic and Adsorption Studies

Activation energy of adsorption (Ea) of mild steel corrosion in 1.0 MHCl was calculated from a plot of log CR vs 1/T according to theArrhenius equation represented as follows:

$\begin{matrix}{{\log CR} = {{- \frac{E_{a}}{{2.3}03RT}} + {\log A}}} & (12)\end{matrix}$where R=ideal gas constant, A=Arrhenius pre-exponential factor, andT=absolute temperature. Other thermodynamic parameters includingstandard enthalpy (ΔH°) and standard entropy (ΔS°) were furthercalculated by plotting log CR/T vs 1/T according to the equation:

$\begin{matrix}{{\log\frac{CR}{T}} = {\frac{\Delta H^{o}}{{2.3}03RT} + \left\lbrack {{\log\frac{R}{Nh}} + \frac{\Delta S^{o}}{2.303R}} \right\rbrack}} & (13)\end{matrix}$where h=Planck's constant and N=Avogadro number.

Calculated values of E_(a), ΔS° and ΔH° of mild steel corrosion in 1.0 MHCl in the absence and presence of inhibitor molecules are presented inTable 4. From thermodynamic point of view, activation energy plays animportant role in understanding the mechanism of action of corrosioninhibitors. Lower Ea values implies higher tendency of the studiedsystem to undergo corrosion. Activation energy increases upon additionof the inhibitor molecules compared to the un-inhibited solution whichindicates an increase in surface coverage and a consequent decrease incorrosion rate. A corresponding increase in enthalpy was also observedwith increasing inhibitor concentration which suggest that the observeddecrease in corrosion rate of mild steel in the presence of theinhibitor molecules is mainly controlled by kinetic parameters [A.Hamdy, N. S. El-Gendy, Thermodynamic, adsorption and electrochemicalstudies for corrosion inhibition of carbon steel by henna extract inacid medium, Egyptian Journal of Petroleum, 22 (2013) 17-25]. A gradualincrease in entropy upon addition of the inhibitor molecules indicates adecrease in disorderliness and a consequent decrease in corrosion rate.

TABLE 4 Thermodynamic activation parameters for mild steel corrosion in1.0M HCl in the absence and presence of inhibitor moleculesThermodynamic Medium parameters Blank 20 ppm 50 ppm 100 ppm E_(ads)(KJ/mol) 13.96 21.51 25.41 29.59 ΔH*_(ads) (KJ/mol) 11.65 18.98 24.1329.02 ΔS*_(ads) (J/mol · K) −110.15 −82.63 −67.25 −50.61

An important feature of organic corrosion inhibitors is their ability toadsorb on metallic surfaces forming a protective film. Adsorption is asurface phenomenon and a consequence of surface energy. Solids areusually in a state of strain which leads to unbalanced residual forceson their surfaces. These forces cause solids to have high surfaceenergies and hence they have the tendency to attract and retainmolecular species with which they come in contact, a phenomenon known asadsorption. Adsorption could be through electrostatic attractionsbetween charged molecules and the metal ions or through electrontransfer and/or sharing between the molecules and the metal ions. Theformer is known as physisorption while the latter is known aschemisorption. Results from the present study were fitted into threeadsorption models; Langmuir, Frumkin and Temkin and were found to fitthe Langmuir adsorption model with Gibb's free energy of adsorption ofthe inhibitor calculated according to the equation:ΔG _(ads) =−RT ln(55.5K _(ads))  (14)where R is the molar gas constant, T the absolute temperature and 55.5the molar concentration of water [D. Daoud, T. Douadi, H. Hamani, S.Chafaa, M. Al-Noaimi, Corrosion inhibition of mild steel by two newS-heterocyclic compounds in 1 M HCl: Experimental and computationalstudy, Corrosion Science, 94 (2015) 21-37]. The results suggest electronsharing between the inhibitor molecules and iron on the surface of mildsteel to form a strong coordinate type of bond (chemisorption) [I. B.Obot, N. O. Obi-Egbedi, Theoretical study of benzimidazole and itsderivatives and their potential activity as corrosion inhibitors,Corrosion Science, 52 (2010) 657-660, incorporated herein by referencein its entirety].

To further study the adsorption of the inhibitor molecules, mild steelspecimen was immersed in a solution containing 100 ppm PU at 25° C. for24 h, followed by rinsing with anhydrous ethanol and drying in a streamof nitrogen. Contact angle measurement was carried out on bare specimenand specimen immersed in PU. The results confirms the adsorption of PUonto mild steel as the wettability was found to change from a contactangle of 75° (bare steel) to 60° (steel_PU). Thereafter the thinadsorbed film formed on the steel specimen was scratched off andanalyzed by FT-IR. The results shown in FIG. 9 reveal the presence ofthe polymer on mild steel specimen as all peaks assigned to the variousfunctional groups in PU (FIG. 4A) were clearly visible. In addition,absorption band due to −NH wag at 700 cm⁻¹ appeared broadened andincreased in intensity due to bonding to Fe ions present in mild steel[S. A. Rounaghi, D. E. P. Vanpoucke, H. Eshghi, S. Scudino, E. Esmaeili,S. Oswald, J. Eckert, Mechanochemical synthesis of nanostructured metalnitrides, carbonitrides and carbon nitride: a combined theoretical andexperimental study, Physical Chemistry Chemical Physics, 19 (2017)12414-12424, incorporated herein by reference in its entirety], which isin agreement with the proposed chemisorption mechanism of adsorption ofthe polymer on mild steel.

Example 12

Spectroscopic Studies

Mild steel specimens were immersed in 1.0 M HCl in the absence andpresence of 100 ppm PU at 25° C. for 24 h. Thereafter the specimens wererinsed with anhydrous ethanol and dried in a stream of nitrogen and SEMmicrographs were taken as shown in FIGS. 10A-C. Morphology of thespecimens before immersion shows no corrosion except polishing linesthat were clearly visible. Specimens immersed in 1.0 M HCl withoutinhibitor molecules shows evidence of pitting corrosion due to acidattack as shown in the Figure. However, these pits were absent on thespecimens immersed in a solution containing 1.0 M HCl and inhibitormolecules indicating the formation of protective inhibitor films on mildsteel surface which isolates the metal from the corrosive solution.

XPS analysis was further carried out to investigate the adsorption of PUon mild steel specimens and the composition of the films formed. Thehigh resolution XPS spectra of nitrogen (N1s), oxygen (O1s), carbon(C1s) and iron (Fe2p) are presented in FIGS. 11A-D. N1s (FIG. 11A)spectrum after deconvolution was fitted into two distinct peaks at 399.2eV and 400.0 eV. The peak at 399.2 eV was assigned to pyrrolic nitrogenatoms which serve as the basic point of interaction between PU and Fe onmild steel surface (Fe—N) [Y. Tang, F. Zhang, S. Hu, Z. Cao, Z. Wu, W.Jing, Novel benzimidazole derivatives as corrosion inhibitors of mildsteel in the acidic media. Part I: Gravimetric, electrochemical, SEM andXPS studies, Corrosion Science, 74 (2013) 271-282, incorporated hereinby reference in its entirety]. The second peak at 400.0 eV representsquaternary nitrogen atoms which forms as a result of protonation of thepolymer in an acidic medium [T. Jafari, E. Moharreri, P. Toloueinia, A.S. Amin, S. Sahoo, N. Khakpash, I. Noshadi, S. P. Alpay, S. L. Suib,Microwave-assisted synthesis of amine functionalized mesoporouspolydivinylbenzene for CO₂ adsorption, Journal of CO₂Utilization, 19(2017) 79-90, incorporated herein by reference in its entirety]. O1s(FIG. 11B) spectrum was fitted into four distinct peaks at 529.0 eV,530.5 eV, 531.5 eV and 532.0 eV. The peak observed at 529.0 eV wasassigned to oxygen bonded to iron (II) in the form of FeO, while thepeak at 530.5 eV represent characteristic peak of α-Fe₂O₃ [D. K.Bandgar, S. T. Navale, M. Naushad, R. S. Mane, F. J. Stadler, V. B.Patil, Ultra-sensitive polyaniline-iron oxide nanocomposite roomtemperature flexible ammonia sensor, RSC Advances, 5 (2015) 68964-68971,incorporated herein by reference in its entirety]. Others are peaks at531.5 eV and 532.0 eV representing surface oxygen as a result of waterchemisorption to form Fe(OH)₂, and oxygen atoms present in carbonylfunctionality (C═O) of the polymer, respectively [D. K. Bandgar, S. T.Navale, M. Naushad, R. S. Mane, F. J. Stadler, V. B. Patil,Ultra-sensitive polyaniline-iron oxide nanocomposite room temperatureflexible ammonia sensor, RSC Advances, 5 (2015) 68964-68971,incorporated herein by reference in its entirety]. C1s (FIG. 11C)spectrum was fitted into three peaks at 284.5 eV, 285.5 eV and 288.5 eVand were assigned to sp²-hybridized carbon atoms of the phenyl ring,sp³-hybridized carbon atoms and carbonyl atoms present in the polymer,respectively [G. P. Cicileo, B. M. Rosales, F. e. Varela, J. R. Vilche,Comparative study of organic inhibitors of coppercorrosion, CorrosionScience, 41 (1999) 1359-1375, incorporated herein by reference in itsentirety]. Finally, Fe2p (FIG. 11D) spectrum consist of three peaks at711.0 eV (Fe2p_(3/2)), 725.0 eV (Fe2p_(1/2)) and a satellite peak at719.0 eV representing α-Fe₂O₃ [M.-H. Pham, C.-T. Dinh, G.-T. Vuong,N.-D. Ta, T.-O. Do, Visible light induced hydrogen generation using ahollow photocatalyst with two cocatalysts separated on two surfacesides, Physical Chemistry Chemical Physics, 16 (2014) 5937-5941,incorporated herein by reference in its entirety]. The fitted Fe2pspectrum reveal the presence of Fe in the form of ferric, Fe³⁺ (712.5eV) and ferrous, Fe²⁺ (709.0 eV and 710.0 eV) species almost at the sameproportion. The presence of ferric species is due to the oxidation offerrous species of iron during the course of corrosion of the steelspecimen, and the overall XPS results are in agreement with experimentaldata which confirms the adsorption of PU molecules on the surface ofmild steel.

Results of the present disclosure were compared to other recentlyreported polymeric corrosion inhibitors as shown in Table 5. Arelatively higher inhibitive performance was achieved at a lowconcentration of 100 ppm, making PU a potential corrosion inhibitor ofmild steel in industries where acid solutions are used for scaleremoval, acid pickling of metals and oil well acidification.

TABLE 5 Comparison of experimental data with reported results fromliterature Inhibitor Test Conc. % Polymer medium (ppm) Inhibition Ref.Polyurethane 0.5M 1600 99.2 S. Kumar et al. H₂SO₄ Scientific Reports 6(2016) 30937 [29] Polyurethane 0.5M 20 97.0 S. Banerjee et al. RSC H₂SO₄Advances 1(2) (2011) 199-210 Polyether Sea 150 92.0 G. Liu et al. waterDesalination 419 (2017)133-140 Polyaspartic Sea 125 90.2 M. A. Migahedet al. J. acid water Molecular Liquids 224 (2016) 849-858 PVPMA* 1M HCl2000 83.0 R. Karthi et al. Arabian Journal of Chemistry 10 (2017)S627-S635 Polyurea 1M HCl 100 99.9 Present disclosure

Example 13

A water soluble polyurea composition was synthesized and utilized as aninhibitor for mild steel corrosion in acidic medium. Corrosioninhibition studies were carried out using electrochemical techniques andsurface characterization. Structural features of the synthesizedpolyurea derived from DFT calculations were also studied, whichcorrelate with experimental findings. Electronic and structuralcalculations showed that the polymer having high electron densitycentered on the nitrogen, oxygen and pi-system possess the tendency toform a strong interaction with iron forming a stable protective filmwhich prevents the diffusion of oxygen and water molecules to thesurface. Potentiodynamic and electrochemical impedance studies revealedthat PU adsorb onto mild steel through chemisorption interaction andwere correlated with spectroscopic studies. Overall, an inhibitiveefficiency of 100% was achieved at 100 ppm with corrosion current ofless than 1 μA, making PU a suitable molecule for the inhibition of mildsteel corrosion in industries where acid solutions are used for scaleremoval, acid pickling of metals and oil well acidification.

The invention claimed is:
 1. A corrosion inhibition method for ametallic substrate, comprising: immersing the metallic substrate in anaqueous corrosive medium which comprises at least one acid selected fromthe group consisting of hydrochloric acid, sulfuric acid, nitric acid,hydrofluoric acid, and phosphoric acid, introducing a formulationcomprising a polyurea pre-dissolved in a polar aprotic solvent into theaqueous corrosive medium in contact with the metallic substrate,wherein: the formulation is devoid of epoxy ester resin; the polyureacomprises reacted units of a diaminoalkane and a diisocyanate; and thepolar aprotic solvent is at least one selected from the group consistingof N-methyl-2-pyrrolidone, dimethylformamide, dimethyl sulfoxide, and1,3-dimethyl-2-imidazolidinone.
 2. The method of claim 1, wherein thediaminoalkane is at least one selected from the group consisting ofethylenediamine, 1,2-diaminopropane, 1,4-diaminobutane, and1,6-diaminohexane.
 3. The method of claim 1, wherein the diaminoalkaneis 1,4-diaminobutane.
 4. The method of claim 1, wherein the diisocyanateis toluene 2,4-diisocyanate, toluene-2,6-diisocyanate, or both.
 5. Themethod of claim 1, wherein the polar aprotic solvent isN-methyl-2-pyrrolidone.
 6. The method of claim 1, wherein a volume ratioof the polar aprotic solvent to the aqueous corrosive medium is in arange of 1:80 to 1:1,000.
 7. The method of claim 1, wherein the polyureais introduced into the aqueous corrosive medium at a concentration of5-500 ppm. steel.
 8. The method of claim 1, wherein the metallicsubstrate comprises steel.
 9. The method of claim 1, wherein themetallic substrate comprises carbon steel.
 10. The method of claim 1,wherein the aqueous corrosive medium comprises hydrochloric acid. 11.The method of claim 1, wherein the aqueous corrosive medium has a pH of5 or below.
 12. The method of claim 1, wherein the formulation issoluble in water.